"She was a massive influence to say, 'No, put it all out there. Trust yourself. People will really connect with this,'" says the actor, who played Draco Malfoy
The special premieres on Jan. 1
Saoirse Ronan, Emma Watson, Meryl Streep and more star in adaptation of beloved novel
Peter Travers says Darren Aronofsky's unique biblical epic is "something no movie enthusiast would want to miss"